<template>
  <ul class="list">
     <li class="item" 
     v-for="item in letters" 
     :key="item"
     :ref ="item"
     @touchstart="handleTouchStart"
     @click="handleLetterClick"
     @touchmove="handleTouchMove"
     @touchend ="handleTouchEnd"
     >
     {{item }}
     
     </li>
  </ul>
</template>
<script>
export default {
  name: "CityAlphabet",
  props:{
      cities:Object
  },
  computed:{
    letters(){
      const letters = []
      for (let i in this.cities){
           letters.push(i)
      }
      return letters 
    }
  },
  data(){
    return {
      touchstatus:false,
      startY:0,
      timer:null
    }
  },
  updated (){
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods:{
    handleLetterClick(e){
      this.$emit('change',e.target.innerText)
    },
    handleTouchStart (){
         this.touchstatus = true
    }, 
    handleTouchMove(e){
         if(this.touchstatus){
           if(this.timer){
             clearTimeout(this.timer)
           }
           this.timer = setTimeout(() =>{
              const touchY = e.touches[0].clientY - 79
            const index = Math.floor((touchY-this.startY)/20)
            if (index >= 0 && index <this.letters.length){
              this.$emit('change',this.letters[index])
            }
           },16)
         } 
    },
    handleTouchEnd(){
       this.touchstatus=false  
    }
  }
};
</script>
<style lang="stylus" scoped>
  .list
    display: flex
    flex-direction: column 
    justify-content: center
    position: absolute
    top:1.58rem
    right :0
    bottom: 0
    width: .4rem
   .item
     text-align: center
     line-height: .4rem 
     color: #00bcd4;
</style>
